Previously known carburetors for small engines have a flexible diaphragm disposed in a fuel chamber which supplies fuel to both main and idling nozzles or orifices opening into a venturi through which combustion air flows when the engine is operating. Typically, the other side of the diaphragm is subjected to atmospheric pressure and in operation controls a fuel inlet valve disposed between the fuel chamber and a supply of fuel to regulate and maintain substantially constant the pressure of fuel in the fuel chamber.
In operation, when a throttle valve in the throttle bore is rapidly moved from an idle to a wide open position, the engine initially momentarily stumbles because the fuel mixture is too lean. When the engine is operating at wide open throttle, fuel bleeds or is removed from the idle circuit of the carburetor. Consequently, when the engine goes from full throttle to idle, it frequently stumbles and sometimes stalls because the idle circuit supplies insufficient fuel to the engine. Furthermore, when operating at part throttle, the carburetor supplies a fuel mixture which is richer than the ideal mixture for operation of the engine. There are also substantial variations in the level or ratio of carbon monoxide in the exhaust under varying engine load conditions.